1. Field of Invention
This invention relates to an apparatus for pressure processes for coating, stripping, cleaning, and/or drying workpieces using gases, liquids, and/or supercritical fluids, collectively referred to hereafter as “fluids,” preferably that minimize surface tension effects. More particularly, it relates to an apparatus for processing workpieces comprising integrated circuits (IC) or microelectromechanical structures (MEMS) on wafer-like substrates such as Si wafers.
2. Description of Prior Art
In the manufacture of integrated circuit (IC) and microelectromechanical structure (MEMS) devices and other high-value components, treatment of workpieces with heated and pressurized gases and supercritical fluids have been shown by others to be advantageous in several of the processing steps.
The production of integrated circuits with very fine, high-density circuit patterns, for example with pattern widths of less than 100 nm, is hindered by the difficulty of wetting the surfaces of the features during production and wash steps. Furthermore, surface tension and related capillary effects can cause pattern collapse during the drying of very fine IC devices. After processing such an IC and washing it with water, the capillary forces that occur on the IC surface during drying can crush the fine features on the surface (Purtell, 1998). A related problem caused by surface tension effects has long been recognized in the production of MEMS devices (Jafri, Moritz, Busta, & Walsh, 1999; Maboudian & Howe, 1997; Mulhern, Soane, & Howe, 1993).
MEMS, sometimes referred to as micromachines, are tiny devices that are manufactured in bulk, typically through masking and etching processes on the surface of silicon wafers. One crucial step in their production is cleaning and drying them after fabrication processing but before packaging. Water-based cleaning procedures work well until the MEMS are dried. As the water evaporates during the drying process, its high surface tension causes it to pull at the delicate features, breaking them or sticking them to the surface (the so-called “stiction” problem). MEMS manufacturers have turned to several manufacturing solutions including the use of supercritical carbon dioxide dryers to prevent the productivity losses caused by stiction.
To make it supercritical, carbon dioxide gas is heated from room temperature to above its critical point (31° C. or 88° F.) and pressurized to above its critical pressure (73 atmospheres or 1073 psi). Under these conditions, carbon dioxide is a supercritical fluid with liquid-like density and solvent power, but with gas-like diffusibility. For the purposes of MEMS drying, the salient features of supercritical carbon dioxide include its zero surface tension and its ability to dissolve away alcohols and organic contaminants from the surface of the device.
In a typical supercritical carbon dioxide drying method, the MEMS workpiece is processed by usual and customary methods up to and including a final wash with water. Following the wash step, however, it is not dried. It is, instead, immersed in methanol to displace the water. Still wet with methanol, the MEMS workpiece is placed inside a pressurizable chamber which is then filled with supercritical carbon dioxide. The methanol is solvated by the carbon dioxide fluid and dissolved away from the workpiece surface, then the chamber is depressurized, forming gas phase carbon dioxide and leaving a clean, dry MEMS surface.
Due to the near-zero surface tension of the fluid, supercritical carbon dioxide processing method for drying ultrafine IC or MEMS devices is gentler on the surface features than other (wet) methods, potentially lowering the failure rate of manufactured devices (Jafri et al., 1999).
Extending the early work by Mulhern, et al., that described the process of drying MEMS devices using supercritical carbon dioxide (Mulhern et al., 1993), U.S. Pat. No. 5,482,564 issued to Douglas and Wallace teaches a method in which supercritical fluid mixtures can be used to unstick MEMS components.
U.S. Pat. No. 6,048,494 issued to Annapragada discloses a pressure chamber, referred to therein as an autoclave, with a moving cassette inside the pressure chamber and with a movable door inside the chamber that is used to seal the opening through which the chamber is accessed and allow its pressurization.
U.S. Pat. No. 6,067,728 issued to Farmer, et al., discloses a system for drying a microelectronic structure on a wafer substrate using the supercritical fluid method and a chamber with a hinged top and cam plate and roller system to lock the hinged top onto the pressure chamber.
U.S. Pat. No. 5,979,306 issued to Fujikawa, et al., discloses an apparatus for heating and processing a workpiece under high-pressure comprising a vessel which is pressed closed in the axial direction to seal the workpiece within the vessel. The vessel has a sealing means mounted into the lower portion of the vessel to allow formation of a pressurizable processing space, a ram to press the lower portion of the vessel against the upper part of the vessel, and a window frame form to hold the components.
As is evident from the related prior art, processing of IC and MEMS wafers, as well as other high-value workpieces, in heated, pressurized chambers can be used for a variety of fabrication, cleaning, and drying procedures. The previously disclosed pressure chambers used for such processing, however, have moving parts and components that can shed particulates and release them into the apparatus itself and into the room in which the device is located during operation of the apparatus and/or during replacement of internal parts such as the sealing means. The problem with such particulates is that that they lead to higher failure rates of processed workpieces.
Existing devices for processing workpieces under gases or fluids under temperature and pressure control have significant operational and/or maintenance problems with respect to particulate generation and workpiece and/or workroom contamination.